/Engine/Source/Runtime/GeometryCore/Public/Spatial/FastWinding.h
Fast approximation of winding number using far-field approximations. On a closed mesh the winding number will be 1 or more inside (depending on number of "winds"). Outside a closed mesh the winding number will be zero. On an open mesh, the above holds near the mesh but in the "hole" areas the value will smoothly blend from 1 to 0 over a band of width dependent on the hole extent
double FastWindingNumber ( const FVector3d & P )